Food wrap films and cling films are widely used in homes, restaurants, supermarkets, etc. for food preservation in a refrigerator or a freezer, for cooking in a microwave oven or for delivering dishes to a customer's house.
Food wrapping is for prevention of drying (for water retention), dissipation of flavor, absorption of odors from other foodstuffs, contamination with dust, and spilling or spurting of a foodstuff from a container during preservation, microwave cooking or delivery.
Food wrap films for these uses usually have a roll form around a paper winding tube having a width of from 20 to 54 cm to a roll length of from 20 to 100 m and are put in a paper carton box having a saw blade at the upper end of its side. The film is unrolled to a necessary length and cut off by the saw blade.
An iron plate having a thickness of from about 0.2 to 0.3 mm and cut to have a saw-toothed edge is usually used as the saw blade. Recently, a rigid paperboard cut to have a sawtoothed edge is also used. The saw blade, whether made of iron or paper, has a very simple cutting mechanism.
The above-mentioned wrap film is required to have seethrough transparency, resistance to being torn when unrolled, to have self-tack, to have sufficient heat resistance so as not to be fused when used for microwave cooking, and to have satisfactory tearability when cut with a saw blade (hereinafter referred to as cutting properties).
Known materials of wrap films include biaxially stretched vinylidene chloride resins, polyvinyl chloride, and polyethylene resins containing a surface active agent (see JP-B-38-4174, JP-B-43-8606 (the term "JP-B" as used herein means an "examined published Japanese patent application"), and U.S. Pat. No. 4,493,867).
Resin materials comprising chlorine-containing monomers, such as vinyl chloride or vinylidene chloride, involve various problems of food sanitation and environmental conservation, such as suspicion of carcinogenity, migration of plasticizers commonly used in soft vinyl chloride resins into foodstuffs, possibility of generation of dioxin after disposal into soil or on combustion, generation of a chlorine-containing gas on combustion which is one of the causes of acid rain, and the like.
Further, wrap films comprising vinylidene chloride resins or polyvinyl chloride have a disadvantage that an initial tear on cutting propagates to an undesired oblique direction not along the saw blade.
Wrap films made of an ethylene-based resin are free from the problems on combustion. They are easily stretched, but are hard to cut with a saw blade. So its cutting requires a considerable pulling force, the film cannot be cut satisfactorily, or the carton box is deformed. Moreover, they have insufficient heat resistance for microwave cooking and insufficient adhesion to a container, e.g., a PSP tray or a metal or porcelain container, or they are hard to handle due to their limpness. Therefore, polyethylene resin wrap films are incapable of substituting for vinylidene chloride resin films and have a very limited market.
On the other hand, heat-sealable laminated resin films composed of a biaxially stretched polypropylene film having laminated on both sides thereof an ethylene-propylene copolymer film or an ethylene-propylene-butene-1 copolymer film have been proposed as a packaging material for processed foods, such as ham, wiener, bacon, and flakes of dried bonito, fruits, and garments (see JP-A-54-158477 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-B-61-50974, JP-B-61-29377, and JP-B-3-23332).
Where these polypropylene resin laminated films are used as a bag into which coffee beans are sealed and roasted by radiofrequency induction heating in a microwave oven (see JP-A-2-171144) or as a perforated bag in which wieners are put and warmed as such by heating in a microwave oven, they are required to have improved heat resistance.
As laminated films exhibiting sufficient heat resistance withstanding microwave cooking, there has been proposed a film comprising an equal weight mixture of poly(4-methylpentene-1) and a propylene homopolymer (see JP-A-64-70381) and a 10 to 150 .mu.m thick laminated film composed of a polymethylpentene resin layer and a polypropylene or ethylene-propylene copolymer layer (see JP-A-64-53838).
However, the above-described laminated films require a heat-sealing temperature as high as 145.degree. C. or even higher in bag making. Besides, they are too thick and lack flexibility for serving as a wrap film and have poor cutting properties.